1. Field of the Invention
The invention relates to an optical waveguide array for the optical transmission of a signal or energy.
2. Description of the Related Art
Heretofore, one method for producing optical waveguide arrays has used photolithography as described below.
FIGS. 9A-9D illustrate a selective polymerization method as a first example, the method is disclosed in Japanese Patent Laid Open No. 3-143069.
According to this selective polymerization method, a solution of an acryl monomer 52 dispersed in a polycarbonate 51 having a refractive index of 1.59 is cast into a film 55 (FIG. 9A).
Next, a transparent photomask 54 having light shielding portions of optical waveguide patterns is laminated onto the film 55 in close contact therewith. Then, ultraviolet light is radiated onto the film 55 from above and through the photomask 54. As a result, the acryl monomer 52 in each ultraviolet-radiated portion of the film 55 polymerizes with the polycarbonate 51 to form a copolymer 56 in the ultraviolet-radiated portion (FIG. 9B).
The film 55 with the copolymer 56 formed therein selectively is then heated in a vacuum, whereby unreacted acryl monomer 52 is removed from the portions of the film 55 that have not been radiated with the ultraviolet light. Each portion from which the acryl monomer 52 has been removed leaves a polycarbonate portion 57 (refractive index 1.59) behind. As a result, the portions with the copolymer 56 formed therein become lower in refractive index (refractive index 1.575) than the polycarbonate portions 57. Therefore, when an optical waveguide array is later completed, the polycarbonate portions 57 formed in the shape of the optical waveguide patterns serve as core portions, while the portions with the copolymer 56 formed therein become a part of a clad portion (FIG. 9C).
Lastly, the film 55 is sandwiched in between low refractive index materials 58 having a refractive index lower than 1.59 to obtain a filmy, plastic, optical waveguide array 60 (FIG. 9D). The low refractive index materials 58 also constitute a part of the clad portion of the optical waveguide array 60, like the copolymer 56 formed portions. The clad portion consists of the low refractive index materials 58 and the copolymer 56 formed portions, whereby the polycarbonate portions 57, the core portions, are covered completely.
A molding method that provides a second example is illustrated in FIGS. 10A to 10G. Examples of this molding method are disclosed in Japanese Patent Laid Open Nos. 55-120004 and 61-138903.
According to this molding method, first the upper surface of a glass or a metallic flat sheet 61 is coated with a photoresist 62 to form a thin layer onto which is then laminated, in close contact therewith, a photomask 63 having openings defining optical waveguide patterns. Ultraviolet light is selectively radiated onto the layer of the photoresist 62 from above and through the photomask 63.
Next, the photomask 63 is removed and then the photoresist 62 is removed from each of the ultraviolet-radiated portions using a developer. As a result, at each portion not radiated with the ultraviolet light, the photoresist 62 remains on the flat sheet 61 as a projection 64 of a pattern that is a reverse to the waveguide pattern of each opening of the photomask 63 (FIG. 10B).
Then, the flat sheet 61 is exposed to a solvent capable of dissolving the flat sheet 61 in accordance with a chemical etching method. At this time, the solvent does not come into contact with the portions of the flat sheet 61 where there remain the projections 64 of the photoresist 62 so that those portions of the flat sheet 61 are not dissolved. Thereafter, the projections 64 are removed, whereby grooves 66 are formed in the flat sheet 61 correspondingly to the openings of the optical waveguide patterns formed in the photomask 63. The flat sheet 61 having the thus-formed grooves 66 serves as a master 65 or an optical waveguide array (FIG. 10C).
Next, using the master 65, there is formed a clad base 70 of a light transmitting plastic material. The method for forming the clad base 70 from the master 65 could be a casting method or an injection molding method. In this case, an electroconductive nickel film is formed on the groove-side surface of the master 65 by a sputtering method and a thick nickel layer is formed by a nickel electroforming method. Then, a light-transmitting plastic material of a low refractive index is poured into the master 65, and by separating the master 65 from the thus cast light-transmitting plastic material there is formed a stamper 67 having projections correspondingly to the openings of the optical waveguide patterns (FIG. 10D).
Then, using the stamper 67, there is formed a clad base 70 provided with grooves 69 having optical waveguide patterns in accordance with a known method such as, for example, a casting method or an injection molding method. A light-transmitting plastic material having a certain refractive index is used as the material of the clad base 70 (FIG. 10E).
Next, a light-transmitting plastic material is allowed to flow into the grooves 69 of the clad base 70 from one end of the grooves by using capillary action. This light-transmitting plastic material has a refractive index higher than that of the light-transmitting plastic material used to form the clad base 70. After the grooves 69 have been sufficiently filled with the resin, the resin is cured by the radiation of ultraviolet light, whereby cores 71 are formed in the grooves 69 (FIG. 10F).
Lastly, the whole surface on the side where the cores 71 have been formed is coated uniformly with a light-transmitting plastic material having a refractive index lower than that of the cores 71 to form a clad layer 72. In this way there is obtained an optical waveguide array 80 (FIG. 10G). In this case, a clad is formed by both the base 70 and the clad layer 72 to cover the cores 71 completely.
However, according to the selective polymerization method described as the first example above, it is difficult to enlarge the difference in the refractive index between the copolymer portions 57 and the polycarbonate portions 58. Because the core-clad difference in refractive index cannot be made large leads to the problem that the optical waveguide opening angle of the optical waveguide array 60 formed by those materials also cannot be large. The "opening angle" indicates a spread angle at the points where light is emitted from the exit end of each optical waveguide, namely, an angle .theta. C (=sin-1 (NA)) determined by the number of openings, NA, of the optical waveguide (FIG. 5). That the opening angle cannot be made large means that it is impossible to construct an optical waveguide in a curved form having a small radius of curvature. It also means that, since the receiving angle for the light beam incident on each optical waveguide is small that the transmittable light quantity of the light beam coupled to the optical waveguide is small. Although how to calculate the percent opening is omitted, the opening angle of each optical waveguide of the optical waveguide array 60 formed in the first example is as small as 12.6.degree.. The transmission loss of each optical waveguide formed by this method was 0.2 dB/cm.
According to the second method, the molding method, the optical waveguide array 80 can have a large opening angle because it is possible to freely select the material of the clad base 70 and that of the clad layer 72. For example, as the material of the cores 71 there is used an acrylic resin (trade name: Aronix M210, where the refractive index is 1.54, manufactured by Toa Gosei Chemical Industry Co., Ltd.) which is a photocurable resin, and as the material of the clad base 70 and clad layer 72 there is used an acrylic resin (trade name: Aronix M310, refractive index=1.46, a product of Toa Gosei Chemical Industry Co., Ltd.) which is also a photocurable resin. The opening angle of each optical waveguide in the resulting optical waveguide array 80 is 26.5.degree., which is larger than that in the optical waveguide array of the first example. However, the surface roughness of the inner walls of the grooves of the optical waveguides formed by the etching process is very conspicuous. Consequently, the optical waveguides of the optical waveguide array 80 formed in the second example each have a very large transmission loss and so can transmit a limited amount of light. In general, it is necessary that the surface roughness of an optical system, including optical waveguides and lenses, be kept to a value on the order of 0.01 .mu.m or less.